1. Field of the Invention
The present invention relates to an apparatus for driving a piezoelectric element which is used as a fuel injection valve of a gasoline engine, a diesel engine, or the like.
2. Description of the Related Art
A high response characteristic piezoelectric element is applicable to an actuator of a fuel injection apparatus of a gasoline engine, a diesel engine, and the like (see: Unexamined Japanese Patent Publication (Kokai) Nos. 60-249877 and 62-17338). For example, in a fuel injection apparatus, a pressure chamber is linked to a controlled fuel passage, and a needle as a valve member is provided therebetween. Fuel is supplied from the controlled fuel passage, via a clearance of the needle, to the pressure chamber, and further, fuel is leaked from the pressure chamber, via the above-mentioned clearance, to the controlled fuel passage. In this fuel injection apparatus, a piezoelectric element is provided to expand and contract a volume of the pressure chamber, thereby closing and opening the needle. In the prior art, if this piezoelectric element is expanded and contracted by applying a positive voltage and a negative voltage thereto, respectively, a definite positive voltage is applied to the piezoelectric element to expand it, i.e., contract the volume of the pressure chamber, thereby closing the needle, and a definite negative voltage is applied to the piezoelectric element to contract it, i.e., expand the volume of the pressure chamber, thereby opening the needle.
In the above-mentioned prior art method for driving the piezoelectric element, however, if an opening state of the needle is very short, a large closing force cannot be generated at the needle, which may cause a secondary opening, such as a secondary injection, during a closing state, which will be later explained in more detail.